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Plantwise Technical Factsheet

serrated tussock grass (Nassella trichotoma)

Host plants / species affected
Poaceae (grasses)
N. trichotoma is a perennial, drought-resistant, tussock-forming grass, up to 50 cm high and 25 cm wide at the base. The leaves and culms of mature plants droop and are often bleached; culms up to twice as long as leaves and much branched; leaves emerging from base to form a large tussock, narrow (0.5 mm diam.), hard, tightly rolled, finely serrated; leaf blade linear with pointed tip, 80-500 mm in length; leaf sheath up to 160 mm long, rounded and smooth but serrated at the collar or ligule at the blade/sheath junction; ligule firmly membranous, short (ca 1 mm long), rounded, white, non-hirsute. Inflorescence an open, much-branched panicle, 20-35 cm long, but may be up to 75 cm, with brittle filiform branches; soon detaching, leaving the plants free of inflorescences for most of the year. Florets (spikelets) small and inconspicuous, 5-6 mm long, formed towards end of branchlets, with two reddish-brown to purple bracts (glumes) enclosing the seeds (ca 50 per inflorescence), imparting a purple tinge to the inflorescences. Each 'seed' borne singly on a thin stalk, 1.5-2 mm long, with a single 2-3 cm long, unbranched, filiform, twisted, slightly bent awl at the apex (which remains attached at maturity), and a tuft of white silky hairs at the base. When 'seeds' are ripe, the heads droop conspicuously. Grain 1-1.2 mm long. For drawings of 'seed' and grain, see Reed (1977). Roots diffuse and fibrous, forming a deep root system, down to 20 cm or deeper, making plants difficult to uproot.
Prevention and control
Cultural Control

Grazing by cattle and sheep cannot control N. trichotoma; however, at low weed density (10% of an improved pasture), goats can help to restrict weed population density (Campbell, 1982). Nevertheless, most reports claim that N. trichotoma is unpalatable and that animals will not readily graze it (Wells and de Beer, 1987). N. trichotoma burns readily but is quick to recover, whilst associated species are killed (Healy, 1945). Thus, ideal conditions are created for massive weed infestation because of the large seedbank (Campbell, 1982; Wells and de Beer, 1987). In general, therefore, burning can increase rather than decrease weed populations.

Afforestation has been used in both Australia and New Zealand to control N. trichotoma, especially with Pinus radiata (Healy, 1945; Campbell, 1982). However, it can take up to 6 years to shade-out the weed and prevent flowering, and a further 4 years before the tussocks die (Parsons and Cuthbertson, 1992; Miller, 1998). The planting of hedges or living windbreaks to trap windblown panicles has been practised in both Australia and South Africa (Campbell, 1982; Wells and de Beer, 1987).

Mechanical Control

Chipping, or severing the root system, has successfully been practised in New Zealand, following-on from other control measures, but is mainly used to kill isolated plants (Healy, 1945; Denne, 1988).

Ploughing can also have an impact but only in combination with other techniques (Parsons and Cuthbertson, 1992).

Chemical Control

Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:

Integrated Control

None of the methods outlined above can be used in isolation, and the management focus has now changed from trying to kill millions of seedlings, per hectare, up to 3400 million (Healy, 1945), by blanket coverage with herbicide, to promoting pasture competition and the strategic use of herbicides. Earlier, Wells (1978) in South Africa, had detailed an integrated strategy based on prevention of seeding by removing and burning seed heads, killing mature tussocks by chipping, ploughing or spot-spraying and prevention of seed dispersal by planting windbreaks, keeping stock away at seeding time, maintaining a good plant cover, particularly on the veld, by not burning. Essentially, this strategy is no different from that currently being advocated in Australia (Campbell, 1998; Miller, 1998), where it has been concluded that a plan to limit seed re-invasion must be in place to support other control measures, such as using appropriate growing regimes, fertilizers and spot-spraying. Underpinning this integrated control approach, is the realisation that a competitive environment needs to be maintained to prevent invasion and that C4 native grass species can outcompete C3 species such as N. trichotoma, especially in low fertility soils, but that over-grazing is a major factor in reducing their competitiveness (Badgery et al., 2003).
N. trichotoma causes more reductions in carrying-capacity than any other pasture weed in Australia (Parsons and Cuthbertson, 1992; Campbell, 1998). Similarly, in New Zealand, N. trichotoma can reduce the carry-capacity by up to 90%, leading to the abandonment of sheep farming (Healy, 1945). If forced to eat N. trichotoma, both sheep and cattle may die, due to the rumen becoming blocked by undigested leaves (Campbell, 1998). The annual benefits from control of N. trichotoma in New South Wales, Australia alone, have been put at over AU$40 million (Jones and Vere, 1998), mainly due to losses incurred by the wool and lamb industries.
Summary of invasiveness
Serrated tussock grass (N. trichotoma) has become a serious invasive weed in warm temperate grasslands in Australia, New Zealand and South Africa where it causes significant losses in livestock production due to reductions of up to 97% in the carrying capacity of infested pastures. It is also becoming an important invasive weed of conservation areas, particularly native grasslands. Because of its enormous seed production, efficient dispersal mechanisms and long-lived seed banks, N. trichotoma is difficult to control and has enormous potential to invade and spread further within and between countries.
Related treatment support
External factsheets
AgPest Factsheets, AgResearch, English language
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